1. Field of the Present Invention
The present invention relates to ultra-low permeation hoses and to a method of manufacture thereof. The present invention relates in particular to ultra-low permeation hoses having an increased resistance to permeation by such substances as refrigerants, fuel gases and flame cutting gases, and also having heat resistance and resistance to carbon dioxide permeation (referred to hereinafter as “CO2 permeation resistance”), as well as to a method of manufacturing such hoses.
2. Description of the Related Art
The hoses used for carrying refrigerant in automotive air conditioning units and similar applications generally have an inner tube that includes polyamide resin or modified butyl rubber as a refrigerant barrier layer. The refrigerant used until now has been the hydrofluorocarbon R134a, but R134a has a global warming potential (GWP) relative to CO2 of about 3,100, which represents a major problem. Reducing the leakage of refrigerant from hoses to a level as close to zero as possible has become an important goal for preserving the global environment.
To address this challenge, technology is being developed that enhances the low permeability of refrigerant-carrying hoses by employing a metal vapor-deposited film as the refrigerant barrier layer. For example, JP 2-209224 A describes a low-permeation rubber hose in which a thin film made of a specific metal is formed by a sputtering or ion plating process on the outer peripheral surface of a synthetic resin inner tube. JP 2-209225 A describes a low-permeation rubber hose in which a dry-plated thin film of a metal or a metal compound is formed on the outer peripheral surface of a synthetic resin inner tube.
Rubber hoses which use a metal foil as the refrigerant barrier layer so as to maintain low refrigerant permeability even when the hose is deformed have also been proposed in the art. For example, JP 2-80881 A describes a flexible hose of low permeability to hydrofluorocarbon which includes a laminated film composed of metal foil and plastic film as the barrier layer.
Because R134a has a GWP relative to CO2 of about 3,100, there has been much interest lately in replacing R134a with CO2 as the next generation of refrigerants. However, when CO2 is employed as a refrigerant, the CO2 must be used in the system in a supercritical state (referred also to hereinafter as “supercritical CO2”), thus requiring hoses to have greater heat resistance and CO2 permeation resistance. Employing CO2 as a refrigerant will most likely entail using it in a gaseous state, a liquid state, a solid state and a supercritical state.
In the low-permeation rubber hoses described in JP 2-209224 A and JP 2-209225 A, forming a metal vapor-deposited film enhances the low permeability properties of the refrigerant barrier layer. However, because the metal vapor-deposited film is very thin, it is unable to conform to the rubber hose as the hose undergoes large deformation during use. As a result, pinholes and cracks arise in the metal vapor-deposited film, allowing the refrigerant to leak out. Such hoses are particularly unsuitable as ultra-low permeation hoses when CO2 is the refrigerant.
When the laminated film composed of metal foil and plastic film described in JP 2-80881 A is used, deformation of the rubber hose causes the metal foil to rupture, making it impossible to maintain a low permeability to refrigerants. Moreover, JP 2-80881 A fails to specifically mention a method for bonding the metal foil with the plastic film, the bonding of which is generally difficult, or a method for bonding the nylon tubing serving as the inner layer of the hose with the barrier layer.
JP 2001-165358 A describes a hose of low permeability to refrigerants or the like which has at the interior a refrigerant barrier layer that consists in large part of a metal layer and is capable of conforming to deformation of the hose, as well as a method for manufacturing such a hose. However, the adhesive layer in a metal (aluminum)-strength-retaining layer (nylon) construction bonded using an aromatic polyester adhesive ultimately fails when exposed to supercritical CO2.
A need has thus been felt for ultra-low permeation hoses which are fully capable of withstanding deformation of the rubber hose, have an excellent resistance to permeability by substances such as refrigerants, and in particular have a better heat resistance and CO2 permeation resistance when CO2 is used as the refrigerant. There has also been a need for a concrete method of manufacturing such hoses.